Method of recycling metallic coated scrap pieces

ABSTRACT

A method of recycling metallic coated scrap pieces wherein the coating layer liquidus temperature is lower than the core layer solidus temperature, such as brazing sheet scrap pieces, or metallic coated scrap pieces wherein the upper part of the melting range of the coating layer overlaps the lower part of the melting range of the core layer, by at least partially removing the coating layer from the core layer of the scrap pieces making use of a heat resistant metallurgical vessel having an opening for introducing the scrap pieces into the vessel and an exit for discharging essentially molten alloy. The vessel being rotatable around an axis of rotation.

The invention relates to a method of recycling metallic coated scrappieces, such as brazing sheet scrap pieces, by removing the coatinglayer from the core layer of the scrap pieces. The invention alsorelates to the use of a metallurgical vessel for this recycling method.

Below the invention will be elucidated for brazing sheet scrap, but themethod can be used as well for other types of metallic coating layers ona metal core.

During the production of brazing sheet a plate of an aluminium alloyhaving a relatively low Si content for the core of the brazing sheet ison one or both sides clad by means of roll bonding with a plate of analuminium alloy having a high Si content for the clad layer on the core.This sandwich of core plate and clad plate(s) is subsequently rolled soas to bind the clad layer(s) to the core layer and to produce thebrazing sheet product having a thickness of typically between 0.2 and 3mm, for use in the production of for instance heat exchangers forautomobiles.

During the production of the brazing sheet significant amounts of scrapis produced, for instance the beginnings and ends of the sandwich platesafter each hot or cold rolling operation. Because the scrap containsboth aluminium alloys with a high Si content and aluminium alloys with alow Si content, simple melting of the scrap would result in an aluminiumalloy having a raised Si content as compared to the Si content of thecore, which is to high to be used for producing similar type coreplates, unless diluted with substantial amounts of alloys having a verylow Si content.

Various methods are available to separate the clad alloy from the corealloy in the scrap. One of these methods is described in internationalapplication no. WO 00/67942. According to this method, the clad layer isseparated from the core of the brazing sheet by heating the brazingsheet until the clad layer becomes liquid or semi-liquid but the coreremains solid, and removing the clad layer by mechanical means such as ahand held or an automatically operated scraper. This method however isonly suitable for individual pieces of about 50–75 mm thickness having asurface area per side of for instance 1 to 5 m²; sheet of about 3 mm istoo thin.

It is an object of the invention to provide an efficient method ofrecycling metallic coated scrap pieces, such as brazing sheet scrap, byremoving the metallic coating from the core of the scrap pieces.

It is another object of the invention to provide a method of recyclingmetallic coated scrap such as brazing sheet scrap, with which largeamounts of scrap can be recycled.

It is still another object of the invention to provide a method ofrecycling such scrap with which a wide range of sheet thicknesses can berecycled.

It is yet another object of the invention to provide a method ofrecycling such scrap that is economical on an industrial scale.

It is a further object of the invention to provide recycled core andcoating alloys, which can easily be used for the production of new sheetmaterial.

According to a first aspect of the invention, one or more of theseobjects are reached with a method of recycling metallic coated scrappieces of which the liquidus temperature of the coating layer is lowerthan the solidus temperature of the core layer, such as brazing sheetscrap pieces, or metallic coated scrap pieces of which the upper part ofthe melting range of the coating layer has an overlap with the lowerpart of the melting range of the core layer, by at least partiallyremoving the coating layer from the core layer of the scrap piecesmaking use of a heat-resistant metallurgical vessel having an openingfor introducing the scrap pieces into the vessel and exit means fordischarging essentially molten alloy, the vessel being rotatable aroundan axis of rotation, the method comprising the sequential steps of:

-   -   filling the vessel with a load of scrap pieces;    -   supplying an amount of energy to the vessel such that        essentially all the scrap pieces reach a temperature above the        solidus temperature of the coating layer;    -   rotating the vessel during and/or after the supplying of the        amount of energy for forming essentially molten alloy;    -   removing the essentially molten alloy, having the composition of        the coating layer of the scrap pieces mixed with small amounts        of the core layer, through the exit means of the vessel;    -   removing the remainder of the load from the vessel.

Using this method, a certain amount of energy is introduced for acertain amount of scrap, which amount of energy is just enough to havethe coating layer melted in whole or at least in part. This amount ofenergy should be introduced in a certain limited time period, to preventoxidation, and to minimise diffusion of Si from the coating layer intothe core layer when brazing sheet scrap pieces are recycled. Of coursethe amount of energy has to depend on the percentage of the coatinglayer in the scrap pieces in relation to the total load. It is assumedthat due to the rotation of the vessel, the scrap pieces abrade eachother and thus remove the liquid or semi-liquid coating layer from thecore layer in a fast and efficient manner. It is believed that thecombined effect of temperature and rotation results in a very efficientremoval of the coating layer from the core, without the need foradditional abrading means. It is unavoidable that small amounts of thecore layer will be abraded as well, so the molten alloy will consist fora small part of the core alloy.

Preferably, an amount of energy is supplied to the vessel such thatessentially all the scrap pieces reach a temperature above the liquidustemperature of the coating layer. By introducing such an amount ofenergy, essentially the whole coating layer will melt and be removedfrom the core. Due to the higher temperature it can be expected that themolten alloy will contain higher amounts of the core alloy. Higheramounts of the core alloy could also be present because the liquidustemperature of the coating layer falls into the melting range of thecore layer.

According to a preferred embodiment of the method, the remainder of theload is removed from the vessel by supplying a second amount of energyin the vessel to melt the remainder of the load to form a secondessentially molten alloy, preferably while the vessel is rotated, andremoving the second essentially molten alloy having the composition ofthe core layer of the scrap pieces mixed with the remainder of thecoating layer. When used for brazing sheet scrap, in this way twoseparate streams of essentially molten aluminium alloy are poured out ofthe vessel, a first one with a high Si content mainly originating fromthe clad layer of the brazing sheet scrap, and a second one with arelatively low Si content mainly originating from the core layer of thebrazing sheet scrap. Pouring out the aluminium alloy is an easy way toremove the aluminium alloy from the vessel, and it can be formeddirectly into ingots or piglets. The same holds, mutatis mutandis, forother types of metallic coated scrap.

Preferably, the inside lining of the vessel has been preheated beforethe vessel is filled with the load of metallic coated scrap pieces, forthe recycling of brazing sheet scrap pieces preferably to a temperatureof between 550° and 650° C. and more preferred to a temperature ofbetween 600° and 630° C. In this way the vessel itself already can begiven the temperature necessary to melt the coating layer, and theenergy introduced will be used to heat the scrap pieces and to melt thecoating layer.

According to a preferred embodiment the vessel is tiltable and thevessel is tilted to pour out the essentially molten alloy through theopening. The opening of the vessel thus acts as exit means too and noseparate exit means are necessary in the vessel.

Preferably, burning a fuel, preferably natural gas, with substantiallypure oxygen, generates the energy in the vessel. By burning a fuel withsubstantially pure oxygen it is possible to supply a sufficient amountof energy in a very fast way, and using natural gas gives a very cleancombustion, so the aluminium alloy will not be contaminated by forinstance sulphur from liquid fuels.

According to a preferred embodiment of the method, the vessel is rotatedduring a time period after the amount of energy has been supplied andbefore the first molten alloy is removed. By using an ongoing period ofrotation after the energy has been supplied, and before the first moltenalloy is removed, it is assumed that a better energy distributionthrough the brazing sheet scrap is reached, resulting in a betterremoval of the clad layer from the core layer.

Preferably, a vessel is used having a useful volume of 3–20 m³,preferably of approximately 5 m³, which is rotated with a velocity of0.2–10 rpm. For a vessel having such a volume, this rotational speedresults in a good mixing and abrading of the brazing sheet scrap pieces.

It has been found that the method according to the invention can be usedin particular for recycling brazing sheet scrap pieces having a corelayer of the AA 6xxx or AA 3xxx type aluminium alloy, such as AA 6063,AA 6060, AA 3003, AA 3103 or AA 3005, and a clad layer of the AA 4xxxtype aluminium alloy, such as AA 4343, AA 4047, AA 4004, or AA 4104. Forthese types, the Si content of the core is up to 0.6% and the Si contentof the clad layer is 6.8 to 13%.

For the brazing sheet types mentioned above, the method according to theinvention should preferably be used for scrap pieces having thicknessesof 0.2 to 100 mm, preferably approximately 0.5–15 mm and/orapproximately 40–70 mm. The best results are obtained for thicker scrap,which is scrap having a thickness of about 5–70 mm.

Preferably the scrap pieces have a square surface area per side of up to0.5 m², preferably of 0.01 to 0.25 m²; the pieces thus having dimensionsof approximately 10 to 50 cm. Scrap pieces with these dimensions mixwell in the vessel, whereas too large dimensions may damage therefractory lining of the vessel during rotation and too small pieces ofscrap will presumably melt in total or stick together without anabrading effect.

According to a preferred embodiment of the method, the vessel ispreheated to a temperature of approximately 620° C., the vessel isfilled with a load of 2 to 5 tons brazing sheet scrap pieces, and anamount of energy of 220–260 kilowatt-hour per ton brazing sheet scrappieces, depending on the type and relative thickness of the clad layer,is supplied in the vessel before aluminium alloy is removed from thevessel. These data can be used for the brazing sheet types as mentionedabove and result in a first amount of molten aluminium alloy having a Sicontent of almost half that of the original clad layer; of course partof the core layer of the scrap pieces is molten as well, especially atthe edges of the scrap pieces. The remainder of the scrap will have a Sicontent that is higher than that of the original core layer, but farlower than a total mixture of clad and core layer. This is of coursebecause it is impossible to remove the clad layer completely from thescrap pieces by melting and rotating.

Preferably in the above method, the scrap pieces are heated during atime period of 20 to 50 minutes, preferably approximately 40 minutes. Ithas been found that a heating period of approximately 40 minutes,depending on the load of scrap pieces, is optimal for heating andremoving the clad layer of the scrap pieces. It is not necessary tointroduce an equal amount of energy per minute.

According to a preferred embodiment of the method, a second amount ofenergy of 200–300 kilowatt-hour per ton of the remainder of the load issupplied in the vessel to melt the remainder of the load. The amount ofenergy added must of course be sufficient to melt all the remainder ofthe scrap; preferably the molten core is overheated for furtherprocessing in the casthouse without former solidification.

Preferably, the remainder of the load is heated during a time period of20 to 40 minutes, preferably approximately 30 minutes. This is,depending on the original load, enough time to fully melt the core ofthe scrap pieces.

Optionally fluxing salts are added to the load of brazing sheet scrappieces when filling the vessel. Such fluxing salts and their use arewell known in the art.

According to a second aspect of the invention, use is made of aheat-resistant vessel having an opening and means for introducing energyinto the vessel, the vessel being rotatable around an axis of rotation,and the vessel being preferably tiltable so as to be able to empty thevessel through the opening, for performing the above described methodfor recycling metallic coated scrap pieces. Such vessels are already inuse for processing aluminium-containing dross at a temperature ofapproximately 850° C. It has been found that such vessels can very wellbe used for performing the method according to the present invention. Aparticular suitable metallurgical vessel is given in European patent EP0 627 014 or U.S. Pat. No. 5,527,380, which documents are incorporatedherein by reference.

According to a third aspect of the invention, the core alloy and/or cladalloy as recovered from brazing sheet scrap pieces by using the methodas described above provide an aluminium alloy having a composition whichmakes especially the core alloy suitable for use in the production ofnew brazing sheet. Moreover, the core alloy and clad alloy are producedin a cost-effective and fast manner.

The invention also provides a product made from the core alloy and/orclad alloy as recovered by using the method as described above.

The invention will be illustrated by the following non-limitativeexamples.

EXAMPLE 1

In an industrial scale experiment, a mixture of pieces of brazing sheetscrap having a thickness of 6.5 mm and 14.5 mm has been used, the scrappieces having a clad layer on both sides. The pieces had a length ofapproximately 290 mm and a width in the range of approximately 40 to 90mm. The mass of the scrap pieces having a thickness of 6.5 mm wasapproximately equal to the mass of the scrap pieces having a thicknessof 14.5 mm. The core sheet consisted of an aluminium alloy having a Sicontent of 0.1 to 0.19% and the clad layer of an aluminium alloy havinga Si content of 9.54 to 9.94% on both sides of the core. The nominalpercentage of the clad layer was approximately 24% of the total weight,so 12% on each side of the core layer.

A load of 3295 kg scrap was introduced in an industrial scalemetallurgical vessel as described in European patent 0 627 014, whichwas preheated to a temperature of approximately 620° C. The vessel hadan internal volume of approximately 5 m³. During a time period ofapproximately 33 minutes a total amount of energy of 790 kilowatt-hourwas introduced in the vessel by burning natural gas with substantiallypure oxygen in approximately a 50–50 ratio. During the first 10 minutes,2.2 megawatt was introduced while the vessel rotated at 2 rpm;thereafter during 23 minutes 1.1 megawatt was introduced while thevessel rotated at 0.5 rpm.

After this treatment a total of 1110 kg aluminium alloy was poured outhaving a Si content of 4%.

Subsequently the remainder of the load in the vessel was heated with atotal amount of 670 kilowatt-hour during a time period of approximately25 minutes. First during 9 minutes 2.2 megawatt was introduced while thevessel rotated with 0.5 rpm. Then during 9 minutes 1.43 megawatt wasintroduced while the vessel rotated at 5 rpm, and during the last 7minutes 1.1 megawatt was introduced while the vessel rotated at 0.5 rpm.

After this treatment 1980 kg aluminium alloy was poured out having a Sicontent of 0.67%.

Known fluxing salts were added.

EXAMPLE 2

In an industrial scale experiment, a mixture of pieces of brazing sheetscrap having a thickness of 6.5 mm and 14.5 mm has been used, the scrappieces having a clad layer on both sides. The pieces had a length ofapproximately 290 mm and a width in the range of approximately 40 to 90mm. The mass of the scrap pieces having a thickness of 6.5 mm wasapproximately equal to the mass of the scrap pieces having a thicknessof 14.5 mm. The core sheet consisted of an aluminium alloy having a Sicontent of 0.1 to 0.19% and the clad layer of an aluminium alloy havinga Si content of 9.54 to 9.94% on both sides of the core. The nominalpercentage of the clad layer was approximately 24% of the total weight,so 12% on each side of the core layer.

A load of 4140 kg scrap was introduced in an industrial scalemetallurgical vessel as described in European patent 0 627 014, whichwas preheated to a temperature of approximately 620° C. The vessel hadan internal volume of approximately 5 m³. During a time period ofapproximately 43 minutes a total amount of energy of 970 kilowatt-hourwas introduced in the vessel by burning natural gas with oxygen inapproximately a 50–50 ratio. During the first 10 minutes, 2.2 megawattwas introduced while the vessel rotated at 2 rpm; thereafter during 33minutes 1.1 megawatt was introduced while the vessel rotated at 0.5 rpm.

After this treatment a total of 645 kg aluminium alloy was poured outhaving a Si content of 5.3%.

Subsequently the remainder of the load in the vessel was heated with atotal amount of 788 kilowatt-hour during a time period of approximately30 minutes. First during 10 minutes 2.2 megawatt was introduced whilethe vessel rotated with 0.5 rpm. Then during 10 minutes 1.43 megawattwas introduced while the vessel rotated at 5 rpm, and during the last 10minutes 1.1 megawatt was introduced while the vessel rotated at 0.5 rpm.

After this treatment 3205 kg aluminium alloy was poured out having a Sicontent of 1.1%.

Known fluxing salts were added.

EXAMPLE 3

In an industrial scale experiment, brazing sheet scrap pieces having asize of approximately 100×300×5 mm, were introduced in a vessel asdescribed in European patent 0 627 014, preheated to a temperature ofapproximately 620° C., in a load of 2580 kg, together with known fluxingsalts. A total amount of 469 kilowatt-hour was introduced.

During 25 minutes 208 kilowatt-hour pro ton was introduced, while thevessel was rotated at 0.5 rpm during 10 minutes, then at 1.0 rpm during11 minutes, and at 3.0 rpm during 4 minutes. A first amount of 660 kgaluminium alloy was poured out, having a Si content of 4.92%.

After that during 32 minutes 240 kilowatt-hour pro ton was introduced,while the vessel rotated at 0.5 rpm during 10 minutes, at 2.0 rpm duringthe next 10 minutes, and at 5 rpm during the last 12 minutes. A secondamount of 1860 kg aluminium alloy was poured out, having a Si content of0.41%.

EXAMPLE 4

In an industrial scale experiment, brazing sheet scrap pieces having asize of approximately 300×300×50 mm were introduced in a vessel asdescribed in European patent 0 627 014, preheated to a temperature ofapproximately 620° C., in a load of 4430 kg, together with known fluxingsalts. A total amount of 1034 kilowatt-hour was introduced.

During 41 minutes 245 kilowatt-hour pro ton was introduced, while thevessel was rotated at 0.5 rpm during all 41 minutes. A first amount of1500 kg aluminium alloy was poured out, having a Si content of 4.29%.

After that during 34 minutes 240 kilowatt-hour pro ton was introduced,while the vessel rotated at 0.5 rpm during 10 minutes, at 5 rpm duringthe next 10 minutes, and at 0.5 rpm during the last 14 minutes. A secondamount of 2850 kg aluminium alloy was poured out, having a Si content of0.52%.

EXAMPLE 5

In an industrial scale experiment, a mixture of brazing sheet scrappieces having a thickness of 0.6 mm and a variable size in the order ofmagnitude of 100×300 mm was introduces in a vessel as described inEuropean patent 0.627 014, preheated to a temperature of approximately620° C., in a load of 1465 kg. All scrap pieces have a core of AA 3xxx(Si content less than 0.62%) and a clad layer of AA 4xxx (Si content of7.2–12.65%). A total of 273 kilowatt-hour pro ton was introduced during36 minutes while the vessel rotated at 2 rpm. A first amount of 285 kgaluminium alloy was poured out, having a Si content of 2.04%. A secondamount of 1045 kg aluminium alloy was; after heating, poured out havinga Si content of 1.00%.

Having now fully described the invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade without departing from the spirit and scope of the invention asherein described.

1. Method of recycling metallic coated scrap pieces of which theliqiuidus temperature of a coating layer is lower than the solidustemperature of a core layer, by at least partially removing the coatinglayer from the core layer of said scrap pieces making use of a heatresistant metallurgical vessel having an opening for introducing thescrap pieces into the vessel and an exit for discharging essentiallymolten alloy, the vessel being rotatable around an axis of rotation, themethod comprising the sequential steps of: filling the vessel with aload of scrap pieces; supplying an amount of energy to the vessel suchthat essentially all the scrap pieces reach a temperature above thesolidus temperature of the coating layer; rotating the vessel duringand/or after the supplying of the amount of energy for formingessentially molten alloy; removing the essentially molten alloy, havingthe composition of the coating layer of the scrap pieces mixed withsmall amounts of the core layer, through the exit of the vessel;removing the remainder of the load from the vessel, wherein theremainder of the load is removed from the vessel by supplying a secondamount of energy in the vessel to melt the remainder of the load to forma second essentially molten alloy, and removing the second essentiallymolten alloy having the composition of the core layer of the scrappieces mixed with the remainder of the coating layer.
 2. Methodaccording to claim 1, wherein an amount of energy is supplied to thevessel such that essentially all the scrap pieces reach a temperatureabove the liquidus temperature of the coating layer.
 3. Method accordingto claim 1, wherein the inside lining of the vessel has been preheatedbefore the vessel is filled with the load of metallic coated scrappieces, for the recycling of brazing sheet scrap pieces.
 4. Methodaccording to claim 1, wherein the vessel is tiltable and the vessel istilted to pour out the essentially molten alloy through the opening. 5.Method according to claim 1, wherein the energy in the vessel isgenerated by burning a fuel, with substantially pure oxygen.
 6. Methodaccording to claim 1, wherein the vessel is rotated during a time periodafter the amount of energy has been supplied and before the essentiallymolten alloy is removed.
 7. Method according to claim 1, wherein avessel is used having a useful volume of 3–20 m³, is rotated with avelocity of 0.2–10 rpm.
 8. Method according to claim 1, wherein thescrap pieces are recycled having a core layer of the AA 6xxx or AA 3xxxtype aluminium alloy and a clad layer of the AA 4xxx type aluminiumalloy.
 9. Method according to claim 8, wherein the scrap pieces havethicknesses in the range of 0.2 to 100 mm.
 10. Method according to claim8, wherein the scrap pieces have a square surface area per side of up to0.5 m².
 11. Method according to claim 8, wherein the vessel is preheatedto a temperature of approximately 620° C., the vessel is filled with aload of 2 to 5 tons brazing sheet scrap pieces, and an amount of energyof 220–260 kilowatt hour per ton brazing sheet scrap pieces, dependingon the type and relative thickness of the clad layer, is supplied in thevessel before aluminium alloy is removed from the vessel.
 12. Methodaccording to claim 11, wherein the scrap pieces are heated during a timeperiod of 20 to 50 minutes.
 13. The method according to claim 1, whereinthe metallurgical vessel is a dross-recycling converter.
 14. The methodof claim 1, wherein the metallic coated scrap pieces are selected fromthe group consisting of brazing sheet scrap pieces, or metallic coatedscrap pieces of which the upper part of the melting range of the coatinglayer has an overlap with the lower part of the melting range of thecore layer.
 15. Method according to claim 1, wherein the inside liningof the vessel has been preheated before the vessel is filled with theload of metallic coated scrap pieces, for the recycling of brazing sheetscrap pieces to a temperature of between 550° and 650° C.
 16. Methodaccording to claim 1, wherein the inside lining of the vessel has beenpreheated before the vessel is filled with the load of metallic coatedscrap pieces, for the recycling of brazing sheet scrap pieces to atemperature of between 600° and 630° C.
 17. Method according to claim 1,wherein the energy in the vessel is generated by burning natural gaswith substantially pure oxygen.
 18. Method according to claim 1, whereina vessel is used having a useful volume of approximately 5 m³ is rotatedwith a velocity of 0.2–10 rpm.
 19. Method according to claim 1, whereinbrazing sheet scrap pieces are recycled having a core layer of selectedfrom the group consisting of AA 6063, AA 6060, AA 3003, AA 3103 or AA3005, and a clad layer selected from the group consisting of AA 4343, AA4047, AA 4004, or AA
 4104. 20. Method according to claim 8, wherein thescrap pieces have thicknesses in the range of 0.5–15 mm.
 21. Methodaccording to claim 8, wherein the scrap pieces have thicknesses ofapproximately 40–70 mm.
 22. Method according to claim 8, wherein thescrap pieces have a square surface area per side of 0.01 to 0.25 m². 23.Method according to claim 11, wherein the scrap pieces are heated duringa time period of approximately 40 minutes.
 24. Method of recyclingmetallic coated scrap pieces of which the liqiuidus temperature of acoating layer is lower than the solidus temperature of a core layer, byat least partially removing the coating layer from the core layer ofsaid scrap pieces making use of a heat resistant metallurgical vesselhaving an opening for introducing the scrap pieces into the vessel andan exit for discharging essentially molten alloy, the vessel beingrotatable around an axis of rotation, the method comprising thesequential steps of: filling the vessel with a load of scrap pieces;supplying an amount of energy to the vessel such that essentially allthe scrap pieces reach a temperature above the solidus temperature ofthe coating layer; rotating the vessel during and/or after the supplyingof the amount of energy for forming essentially molten alloy; removingthe essentially molten alloy, having the composition of the coatinglayer of the scrap pieces mixed with small amounts of the core layer,through the exit of the vessel; removing the remainder of the load fromthe vessel; wherein the brazing sheet scrap pieces are recycled having acore layer of the AA 6xxx or AA 3xxx type aluminium alloy and a cladlayer of the AA 4xxx type aluminium alloy and, wherein a second amountof energy of 200–300 kilowatt-hour per ton of the remainder of the loadis supplied in the vessel to melt the remainder of the load.
 25. Methodaccording to claim 24, wherein the remainder of the load is heatedduring a time period of 20 to 40 minutes.
 26. Method according to claim24, wherein fluxing salts are added to the load of scrap pieces whenfilling the vessel.
 27. Method according to claim 24, wherein theremainder of the load is heated during a time period of approximately 30minutes.
 28. The method according to claim 24, wherein the vessel istiltable to be able to empty the vessel through the opening, forperforming the method for recycling metallic coated scrap pieces. 29.Method of recycling metallic coated scrap pieces of which the liguidustemperature of a coating layer is lower than the solidus temperature ofa core layer, by at least partially removing the coating layer from thecore layer of said scrap pieces making use of a heat resistantmetallurgical vessel having an opening for introducing the scrap piecesinto the vessel and an exit for discharging essentially molten alloy,the vessel being rotatable around an axis of rotation, the methodcomprising the sequential steps of: filling the vessel with a load ofscrap pieces; supplying an amount of energy to the vessel such thatessentially all the scrap pieces reach a temperature above the solidustemperature of the coating layer; rotating the vessel during and/orafter the supplying of the amount of energy for forming essentiallymolten alloy; removing the essentially molten alloy, having thecomposition of the coating layer of the scrap pieces mixed with smallamounts of the core layer, through the exit of the vessel; removing theremainder of the load from the vessel, wherein the remainder of the loadis removed from the vessel by supplying a second amount of energy in thevessel to melt the remainder of the load to form a second essentiallymolten alloy, while the vessel is rotated, and removing the secondessentially molten alloy having the composition of the core layer of thescrap pieces mixed with the remainder of the coating layer.